In a traditional down-looking fish finding apparatus, a sonar pulse is transmitted approximately straight down into the water. The surface at the bottom of the body of water usually produces a relatively large and direct upward reflection back to the transducer, and it is thus usually possible to easily identify the reflection caused by the bottom surface. The assumption is made that there is little or nothing between the top surface and the bottom surface except fish, and thus virtually all echoes arriving before the large echo from the bottom surface are treated as detected fish. A few systems discard all echoes arriving within a short predetermined time interval just before the large reflection from the bottom surface, on the theory that they represent weeds or logs on the bottom surface, or irregularities in the bottom surface itself.
The present invention relates to a side-looking fish finding apparatus, or in other words an apparatus which transmits a sonar pulse in an approximately horizontal direction in an attempt to locate objects along a path located just below and extending approximately parallel to the surface of the body of water. A side-looking apparatus faces a significantly more complex situation than a traditional down-looking apparatus. First, there is typically no large and easily identifiable echo from the bottom surface, because the sonar pulse is traveling approximately parallel to the bottom surface and a flat or slightly inclined bottom will reflect most of the pulse so that it continues to propogate in approximately the same horizontal direction. If the water to air interface is smooth, the same phenomena can occur at that interface, which produces a channeling effect commonly referred to as bottom skimming without a large reflection back to the transducer. Thus, only a small portion of the pulse intensity is typically reflected from the bottom surface back toward the transducer, and in fact there may be several weak bottom reflections spaced in time, depending on the shape of the bottom surface. Some of these weak reflections may arrive before reflections from fish of interest. Moreover, a variety of other objects of significant size may be located between the transducer and fish which are to be detected, such as rocks, tree stumps, dock support poles, retaining walls, sand bars, and so forth. These objects can produce relatively large echoes which arrive before echoes from objects of interest such as fish. Weeds growing on the bottom surface may also produce an echo pattern. Consequently, the relatively simple techniques used in down-looking systems, namely treating all echoes received prior to a large bottom echo as fish and ignoring all echoes received subsequent to the bottom echo, simply are not suitable for use in a side-looking system.
Existing side-looking systems are relatively expensive, and typically report virtually all of the echoes which they receive without attempting to discriminate echoes produced by fish from echoes produced by other objects.
A further problem with existing side-looking systems is that, when two or more systems are used in close proximity to each other, they are more likely to interfere with each other than would be the case with traditional down-looking systems, because of the fact that pulses are transmitted horizontally toward other boats rather than directly downwardly. For example, if two units made by the same manufacturer are being used in different boats which are located near each other, and if the periodic transmission of pulses by these units comes into synchronization with each other, one or both of the units could receive not only echoes from its own sonar pulses, but also echoes from sonar pulses produced by the other unit, which could generate false readings.
Accordingly, an object of the present invention is to provide a side-looking apparatus for detecting objects in water which is capable of efficiently discriminating echoes produced by fish from echoes produced by other objects.
A further object is to provide such a system which is relatively inexpensive in comparison to known side-looking systems.
A further object is to provide such a system in which two or more identical units can be simultaneously used in close proximity to each other with little or no risk of erroneous readings.
A further object is to provide such a system which is relatively compact and light, and which is highly reliable.